1. Field of the invention
This invention relates to a sensor having a pair of sensor units, one of which is a detecting unit and the other of which is a reference unit. More particularly, it relates to a moisture, gas or infrared ray sensor having excellent detection ability due to its high sensitivity, excellent response characteristics and lowered power consumption.
2. Description of the prior art
A variety of moisture sensors have been developed. Particularly, sensors for detecting relative humidity in the atmosphere utilize the phenomenon that the electrical resistance or the electrical capacity of a moisture sensitive material varies depending upon a variation in the humidity or water vapor of the atmosphere. As moisture sensitive materials, there have been, for example, a moisture sensitive material having a sintered body of metal oxides such as iron oxide (Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4), tin oxide (SnO.sub.2), etc., or a metal oxide film; a moisture sensitive material having a hydrophilic polymer film or a polyelectrolyte; a moisture sensitive material having an electrolyte salt such as lithium chloride (LiCl); and a moisture sensitive material having a hygroscopic resin or polymer film in which conductive particles or fibers such as carbon are dispersed. However, the above-mentioned sensors have difficulties in detecting a slight variation in water vapor of the atmosphere such as that in an operating microwave oven under severe conditions in which the temperature of the atmosphere drastically changes. The reasons are as follows: Provided that the amount of water vapor of the atmosphere to be detected is maintained at a fixed level, when the temperature alone of the atmosphere rises, the relative humidity of the atmosphere is lowered in relation to the saturated water vapor pressure. Moreover, when the temperature of the atmosphere drastically rises, even though a slight increase in water vapor arises, an increase in the relative humidity of the atmosphere is offset by the change in temperature, or a decrease in the relative humidity is affected. As a result, substantial variation in water vapor cannot detected. Therefore, for the detection of humidity of certain environments, the detection of absolute humidity (i.e., an amount of water vapor) is advantageous over that of relative humidity.
As a detecting means for the detection of absolute humidity, there have been apparatuses utilizing the decay of microwaves due to water vapor or absorption of infrared rays due to water vapor. Although these apparatuses are superior in that a slight variation in the amount of water vapor can be detected even under severe conditions where the above-mentioned drastic changes in temperature arise, the structure of such a temperature compensation means becomes unavoidably large and results in a high production cost. On the other hand, there has been a thermal conduction type moisture sensor provided with a pair of thermistors for the detection of absolute humidity utilizing a difference in the thermal conductivity between the wet air and the dry air. This moisture sensor is compact and exhibits a moisture-detecting ability even under severe conditions where drastic changes in temperature arise, but it cannot produce sufficient output power based on a slight variation in the amount of water vapor so that it is inferior in detection sensitivity and response speed.